This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Our research program concerns the principles of macromolecular assembly and the organization of large protein and protein-nucleic-acid complexes. During the current period of work at NE-CAT, our focus has been on the following problems. (a) Structure of rotavirus, as it relates to virus assembly and viral entry. (b) Receptor binding by the surface glycoprotein of New World arenaviruses and the way variation in viral protein sequence affects host range. (c) Conformational states of the HIV envelope protein. (d) Structures of kinetochore-associated protein complexes.